(1) Field of the Invention
The present invention generally relates to an ink jet recording method and head, and more particularly to an ink jet recording method and head in which a dot is recorded using one or a plurality of ink droplets so that the size of the dot is controlled.
(2) Description of the Related Art
A non-impact recording method is advantageous since a noise level generated during a recording process is low enough to be ignored. Particularly, an ink jet recording method, which is one example of the non-impact recording method, can make prints at a high velocity and can make prints on normal sheet without an image fixing process. Since, the ink jet recording method is a very useful recording method, printers using the ink jet recording method have been proposed and have been put into practical use.
In such an ink jet recording method, droplets of recording liquid named as ink are jetted, the ink droplets are adhered to the recording medium and images are formed on the recording medium by the adhered ink droplets. The ink jet recording method is disclosed, for example, in Japanese Patent Publication No.56-9429. In the method disclosed therein, a bubble is generated in the ink in a liquid chamber by heating the ink so that pressure in the ink is increased. The ink is then jetted, as an ink droplet, from a fine orifice at the lead end of a nozzle and an ink dot is recorded on the recording medium.
Various method have been proposed based on the above principle of the ink jet recording method. For example, Japanese Laid Open Patent Application No.59-207265 discloses a method by which gray scale images are recorded. In this method, a sequence of pulses is supplied to a heater so that ink droplets are generated, a single droplet into which the generated ink droplets are connected is jetted to a recording medium, and a single dot is formed on a recording medium. The number of the generated ink droplets is controlled in accordance with the number of pulses included in a sequence of pulses.
A method disclosed in Japanese Laid Open Patent Application No.63-53052 has been known. In this method, a gray scale image is recorded by jetting a sequence of ink droplets which are to be fused into a single dot on a recording medium within a wet time of the recording medium. That is, ink droplets are separately jetted at a high velocity and reached to a recording medium, and the ink droplets are then fused into a single dot on the recording medium within the wet time of the recording medium. The size of the dot on the medium corresponds to the number of ink droplets fused into the single dot within the wet time of the recording medium.
Further, a method disclosed in Japanese Patent Publication No.59-43312 has been known. In this method, to improve the output responsibility and stability of ink droplets in response to pulses supplied to a heater to generate bubbles in the ink, an input interval of the pulses in the maximum frequency at which ink droplets are generated is controlled so as to be as large at least three times as the half-width of each pulse.
In the method disclosed in Japanese Laid Open Application No.59-207265, to maintain a condition in which a plurality of jetted ink droplets are connected together to form a single ink droplet, the ink droplets must be jetted at a low velocity. However, if the droplets are jetted at the low velocity, a locus in which each droplet is jetted is not stable, so that deterioration in the quality of prints occurs. In addition, the ink droplets jetted at the low velocity are easily affected by the malfunction of the ink jet recording head and the variation in the moving velocity of the recording head. If the ink jet recording head is moved at a high velocity, a true circular dot is not made on the recording medium when the jetted ink droplets are adhered to the recording medium. As a result, an image formed on the recording medium becomes not clear.
Japanese Laid Open Patent Application No.63-53052 does not disclose conditions under which ink drops are to be jetted other than only a condition in which a time interval separating the activation of the heater to jet the next ink droplet from the disappearance of the bubble falls within a range between 0.1 microsecond and 1.0 millisecond. Thus, it can not be understood under what conditions ink droplets are to be jetted nor how the recording head to be used is to be structured, so that the method can not realized.
Japanese Patent Publication No.59-43312 describes only conditions under which ink droplets can be stably jetted by an on-off operation of a pulse signal. That is, the gray scale printing method is not disclosed in Japanese Patent Publication No.59-43312, but discloses only conditions for a stable binary printing operation.
Accordingly, a general object of the present invention is to provide a novel and useful ink jet recording method and head in which the disadvantages of the aforementioned prior art are eliminated.
A more specific object of the present invention is to provide an ink jet recording method and head in which a dot size is controlled in accordance with image density information so that gray scale recording of images can be performed.
Another object of the present invention is to provide an ink jet recording method and head in which very small ink droplets can be formed by infinitesimal amount of energy and the gray scale recording of images can be performed by controlling the number of ink droplets so that the dot size is controlled.
Another object of the present invention is to provide an ink jet recording method and head in which the very small ink droplets can be stably jetted at a high frequency.
The above objects of the present invention are achieved by an ink jet recording method for jetting ink droplets from an ink jet recording head to a recording medium and forming a dot image on the recording medium, the ink jet recording head having an ink chamber for storing ink, an ink jetting orifice, an ink path connecting the ink chamber and the ink jetting orifice and a heater element provided in the ink path, the ink jet recording method, comprising the steps of: (a) inputting a set of pulses to the heater element so that the heater element is repeatedly activated by the driving pulses, a number of pulses in the set depending on image information supplied from an external unit; (b) repeatedly generating a bubble in the ink in the ink path in accordance with repeated activation of the heater element; and (c) separately jetting ink droplets from the ink jetting orifice by repeatedly generating the bubble in the ink, a number of the ink droplets being equal to a number of the driving pulses input as a set to the heater element in step (a), the ink droplets jetted from the ink jetting orifice forming a single dot on the recording medium, wherein a time interval at which the driving pulses are input to the heater element is equal to or greater than 4T, T being a time period from a time at which the inputting of the pulses to the heater element starts to a time at which the bubble reaches a maximum size, and each ink droplet is a slender pillar so that a length of each ink droplet is at least three times as great as a diameter thereof.
The above objects of the present invention are also achieved by an ink jet recording head for jetting ink droplets to a recording medium and forming a dot image on the recording medium, the ink jet recording head comprising: an ink chamber for storing ink; an ink jetting orifice from which ink droplets are jetted; an ink path connecting the ink chamber and the ink jetting orifice; and a heater element provided in the ink path, a set of pulses being supplied to the heater element so that the heater element is repeatedly activated by the driving pulses, a bubble being repeatedly generated by the activation of the heater element, the ink droplets being jetted from the ink jetting orifice. by the bubble being repeatedly generated, and the jetted ink droplets forming a single dot on the recording medium, wherein an energy E of each pulse falls within a range of 0.6xc3x9710xe2x88x926-14.8xc3x9710xe2x88x926 (joule), an area S of the ink jetting orifice falls within a range of 2xc3x9710xe2x88x926-5xc3x9710xe2x88x926 (cm2) and a ratio E/S falls within a range of 0.3-3.
According to an ink jet recording method of the present invention, as the ink droplets are separately jetted and each dot is a slender pillar, a fine flying locus of each ink droplet is obtained and a flying velocity of each ink droplet is stable. Thus, a dot image having a high quality can be obtained. In addition, according to an ink jet recording head of the present invention, small ink droplets can be stably jetted from each ink jetting orifices.
Additional objects, features and advantages of the present invention will become apparent from the following detailed description when read in conjunction with the accompanying drawings.